A process for the burning of solid or sludge-like fuels in a fluidized bed in which a fluidized bed containing fuel particles having a density significantly in excess of 2 kg/m.sup.3 is separated by a sharp density demarcation from the free space above the bed in which the particle density may be 1 to 2 kg/m.sup.3 and in which combustion in the fluidized bed is effected with primary air and in the free space above the fluidized bed with secondary air is known as the Circofluid Process and is described in a brochure entitled "Circofluid--ein neues Konzept fur Dampferzeuger mit Wirbelschichtfeuerung", i.e. "Circofluid--a new concept for steam generators with fluidized bed firing".
It will be understood that the fluidized bed itself can comprise at least 95% (usually between 97% and 98%) inert particles (e.g. ash particles) with particle size in the vicinity of 0.5 mm. Particles of smaller size may be entrained out of the bed while particles of larger size may settle from the bed and can be removed as ash. The density of the fluidized bed, determined primarily by the density of the inert materials thereof, can be for example 800 to 1000 kg/m.sup.3 and about 10 to 20% less than the bulk density of the materials of the bed. The fuel in the bed, which makes up only a small fraction can be added in a particle size range generally of 8 to 10 mm, although it is not unusual to supply particles of up to 25 mm in size.
In this process, the velocity of the fluidizing air formed by the primary air is so selected that finer particles are entrained out of the fluidized bed and with the combustion gas into the free space above the fluidized bed. In this manner, it is possible to maintain a suspension density in the free space above the fluidized bed at 1 to 2 kg/m.sup.3. The entrained particles, including fine fuel particles with a particle size less than 0.5 mm and usually below 0.4 mm and combustible gas components including volatiles driven off from the fuel in the fluidized bed, burn in the free space.
Since only about 60% of the heat value of the fuel is liberated in the fluidized bed, the heat value liberated in the free space above the fluidized bed is the remainder, namely about 40% of the total heat. Correspondingly, at full loaded operation, approximately 40% of the combustion air must be supplied as secondary air. The fluidized bed temperature in the brochure in the case of the burning of bituminous coal is given as 850.degree. C. It must be held constant within narrow limitations to insure an optimum desulfurization in the fluidized bed.
The temperature in the fluidized bed is controlled by feeding back a metered quantity of cooled flyash. The radiant heating surfaces of the combustion chamber are matched to the heat liberated. The heat transfer at the boiler walls is determined by the size of the effective heating surfaces or their construction and the tamping mass lining the chamber so that the free space temperature remains about 800.degree. to 950.degree. C.
The secondary air proportion is reduced on partial loading of the apparatus since a substantially constant quality of air is required to fluidize the bed and thus a greater proportion of the fuel must be burned in the bed. The proportion of the fuel which burns in the bed is, in turn, dependent upon the characteristics of the fuel, for example, its specific comminution or pulverization characteristics, its expanding or swelling characteristics, its reactivity, its volatiles content and the like. Since a fluidized bed combustion is especially advantageous for ballast-containing fuels, it may be desirable to provide a basic firing with, for example, coal, to which auxiliary fuels may be added, for example, waste gases from a coke oven, sludge from a coal treatment plant or a sewage clarifier, supplied in uncontrollable amounts. Ballast-rich fuels, moreover, show substantial variations in fuel quality.
Because of the variations in loading of the apparatus, changes in fuel quality and variations in the amounts, types and characteristics of auxiliary fuels, the temperature in the free space tends to vary greatly in the earlier system even if the fluidized bed temperature is held more or less constant. When the free space temperature falls too sharply, the ignition of fuel particles entrained from the fluidized bed and thus the final combustion deteriorates. In addition, in such cases, high levels of emissions, carbon monoxide and sulfur dioxide may arise.